This invention relates to a pneumatic actuator system including one or more piston-cylinder type actuators, each having a working piston with a load engaging piston rod. The system further comprises a control circuit with a directional valve for directing pressure air to alternative sides of the working piston of each actuator for accomplishing movement of the working piston in alternative directions, and flow restrictions for restricting the air feed flow to the actual driving side of the working piston.
Actuator systems of this kind are used in the aluminium producing industry, in particular for crust breaking operations in electrolytic alumina reduction pots. Aluminium producing plants are usually big operations having a great number of electrolytic baths for reduction of aluminium oxide into metallic aluminium. For repeatedly breaking the crust layers inevitably formed on top of the electrolytic baths and thereby enabling supply of alumina, i.e. pulverized aluminium oxide into the baths, there are used a great number of big-size pneumatic actuators.
A problem inherent in this type of operations is that the crust layers to be broken may vary in thickness from zero to a very massive crust body, and to be able to deal with the thicker crust layers the actuators have to be big and powerful. For a big aluminium producing plant this creates a demand for a huge pressure air supply capacity, because driving the working piston of each actuator in reciprocating cycles requires a large amount of pressure air. This causes substantial costs, and there is a serious need in this type of industry to reduce the overall pressure air consumption and to bring down these costs
Previously, a solution to this problem has been suggested which means that the current driving side of the actuator working piston is fed with pressure air via a flow restriction, whereas the opposite idling side of the working piston is vented through a substantially unrestricted outlet. This means that the pressure on the driving side of the working piston is quite low as long as the resistance to the piston movement is low, but increases automatically all the way up to the maximum pressure available in case the resistance to piston movement becomes higher.
In the above described field of use for pneumatic actuators, the crust layers are very thin and result in very low piston loads in more than 90% of all crust breaking cycles. In less than 1% of all cycles, the crusts are thick enough to require a full power action. This means that in a vast majority of the crust breaking cycles, the required air pressure behind the working piston is very low, as is the pressure air volume fed into the actuator cylinder. The above described restricted air feed to the actuator means a certain reduction in the consumed pressure air volume compared to previously used full pressure actuator operations, and of course it means a substantial cost saving for the industry. A condition for this, however, is that the piston is allowed to return to its start position immediately after reaching its extended extreme position, otherwise, there will still be a full pressure build-up in the actuator cylinder and a resulting pressure air waste.
Due to reasons as customer requirements and slow signal communication between position sensing means at the electrolytic pot and a control unit, the piston in previous actuators has been maintained for some time in its extended end position, which means that even if you use feed flow restrictions to keep down the drive pressure on the piston during piston movement, there will still be a full pressure build-up in the actuator cylinder after the piston has completed its strokes. Such pressure build-ups are of no use but a waste of expensive pressure air.
The main object of the present invention is to accomplish a pneumatic actuator system by which the pressure air consumption is brought down to a minimum such that no more pressure air than absolutely necessary is spent on the actuator operation while automatically providing maximum pressure and top power capacity when ever required.
Another object of the invention is to provide a pneumatic actuator system having short and quick air communication routes, so as to make the actuator operation distinct and without any delays in relation to given command signals.
A further object of the invention is to enable operation of more than one actuator by a single directional valve.
A still further object of the invention is to provide an actuator system wherein components sensitive to harsh environmental factors like heat, strong magnetic fields, chemically active substances etc. may be located remotely from the actuator without increasing the pressure air consumption.
Other objects and advantages of the invention will appear from the following specification containing a detailed description of preferred embodiments of the invention with reference to the accompanying drawings.